Selective bolometer



- s 2,994,115 'ICC patented Joyas. 19`

2,994,053 SELECTIVE BOLOMETER Russell D. De Waard, Old Greenwich, Conn., assignor to Barnes Engineering Company, Stamford, Conn., l v corporation of Delaware Filed May 24, 1960, Ser. No. 31,437

. Claims. (Cl. 338-18) This .invention relates to improved thermistor bolorncters, Yand more particularly to improved selective irnmersed ther'rnistor bolometers.

'Ihermistorfboiomcters have achieved 'great commerhave the great advantage that they can be used with any cial success as infrared'getectgrs, 'and infact are the standard detectors for the middle and far infrared. They kind of infrared radiation and are not limited to partieu-l `lar regions of the infrared, usually the nearer infrared,

which constitutes so serious-a limitation for photodctectors.

The standard material for thermistors has been a mutture ofmetallic oxides such asnickel or manganese ox- "ides which are fused-to form resistors ofvery high negative temperature coefficients.

' Great as has been the usefulness of thel metal oxide thcrmistors they still leave much to be desired for certain uses, particularly in instruments where infrared ra'- 2 the present invention permits for the rst time produc an immersed thermistor bolometer which can be m: selecitve for certain wavelength hands Ain the infrared. should be understood that immersed thermistor bolol ters are not claimed as such in the present invention they constitute the subject matter of the copentiing plicationof Wormser, Serial No. 459,07o, tiled Septt ber 29, 1954. Improved immersed thermistor bolol ters, however, are included as a specific modification the present invention. t

Essentially the present invention utilizes as thermist instead of oxides the metallic semiconductors germani or silicon in extremely thin films. .The heat condueti and heat diffusivityof these elements is orders of maf tudegreater than the ordinary oxide material norm used, and the volumetric heat'eapacity is less than hal great. Asa result these semiconductor thermistors 'the present invention have time constants in micro onds, and under favorable conditions approach time diation is chopped before it strikes a thermistor bolometer. This permits producing an ALC. signal instead of a D.C. signal, which makes simpler and better electronic circuits possible. ln order to respond to interrupzed radiation the thermistor in the bolometer has to be heated and cooled. The cooling istaken care of by attaching the thermstor in good heat conducting relation with a relatively large backing block of a material of high heat conductivity such as aluminum oxide, beryllium oxide, and metals like copper. The backing block operates as a heat sink and the rate at which the thermistor responds is then determined largely by the heat dilfusivity of the thermistor material. As :1 result there is a definite limitation on the frequency of chopping which is possible. 1n general, even under the best circumstances time constants of a millisecond, or a major fraction of a milliseeond, represent the limit. This should bccompared with the time constants of photoconductive infrared detectors which may be of the order of a microsecond. In other words there is a gap in time constants of several orders of magnitude.

Sensitivity of a thermistor is determined by its temperature coeicient and at 300 K. 4.2 percent resistanceehange per degree, C. is about the limit. This :rather 'modest sensitivity while thoroughly practical for many instruments requires a maximum concentration 'of infrared energy on the thermistor. This has made it necessary to adopt optical configurations of maximum et`- ciency. One of the simplest and best methods of increasing the intensity of infrared. radiation on a thermistor bolometer is to immerse the bolometer, usually in a germanium lens. The enormous refractive index of germanium, four, permits a greatincrease in detector eftcicncy` and even refractive material of more modest re. fractive index, such as silicon, barium titanate, crystalline aluminum oxide, referred to loosely in the art as sapphire, and the like have been used in immersed bc -lometers to produce varied degrees of vincreased response. In the case of refractive material which is an electrical conductor, such as germanium and silicon, the thermis- -tor has to be insulated, which is usually effected by a very thin film of selenium.

The present invention is of particular importance in the field of immersed thermistor bolometers, and greatly widens the tield for detectors o f this type.-v In particular inthe equation V stante of the order of magnitude of those which are at able with infrared photodetectors. An entirely new t of utility for thermistor bolor'neters is vthereby ope and the comparatively large time constant which co1 tuted a limitation for ordinary thermistor material longer'constitutes a bottleneck for instrument desgr The enormous reduction in time constant which is 1 sible with germanium or silicon is obtained withoutV compromise in sensitivity. ln fact there is an incre Germanium has a higher temperature coefiicient than best of the metal oxide materials and silicon is alr twice as high.

While the very short time constants possible with i thin films of silicon or germanium permitcxtrcmcly'l radiation chop frequencies, normally these extremely sl time constants are not necessary. Lower frequencies j mit increased sensitivity ofthe detector either when m tired in terms fo responsivity or detectivity. This cat seen from a consideration of the general' equation detector responsivity which in simplified form, consi ing only the variable quantities and omitting constant as follows:

R: VZ 4mm is the peak bias voltage and Zthe t ma! impedance which is usually in the form of a layer of material of low heat conductivity between thermistor and itsfheat sink. The equation really inch the heat capacity of the thermistor but with thermis of the same material and the same general shape th proportional to thickness designated by t in the -equat Z, of course, is usedV inv its usual sense to designate pedance.

Looking at the equation it will be secrrthat as the queney-is lowered responsivity increases'. .Howeven change in responsivity with changing of t and Z i: important matter and one in which the thermistor bol eters of -the present invention'present a marked ad tage. lfthe frequency is lowered the denominator-,oi fraction is affected only by the product of r and Z the resultsare very different depending on v hether increasedor Z is increased. The numerator involves voltage land impedance'. 'The peak voltage itself is af tion -of'thic'kness and impedance because the heating to the bias voltage must be removed asfast as it i sduced.. As a result voltage is'invcrscly proportion:

. the square root 'of tZ, the time constant If creasing t then responsivity decreasesin proportionto the i square root of t. The situation is not quite asbad with detectivity which is responsivity divided by noise as the noise is also inversely proportional to the square root of i. Therefore, detectivity does not change but there is no `increase.

From the above discussionit will be apparent that'at 16 silicon is transparent'its higher sensitivity may be us lower frequencies for a givenincrease in time constantl the responsivity will be much greater if this increase re' sults from increased thermal impedance. Putting it another way the .thinner the thermistor can be kept and still remain uniform and otherwise electrically satisfactory the greater the increase in responsivity whichis pos-A sible by increasing the time constant. Since'the thermis- 'orsof the present invention can be made much -tbinner f and so with much lower heat capacity than`those hitherto available the invention h as the added advantage that it 'maybe usedl to increase responsivity and detectivity of a thermistor bolome'ter at more moderate frequencies to a greater degree than'can be effected with other materials.

Increased thermal impedanccs, which should be carefully controlled to produce a desired increased time constant, can best be effected by interposing a precisely controlled but thin layer of materialof low thermal conductivity, for example, a film of polyethylene terephthalate. The general production of thermistor bolorneters with con? trolled and increased time constants by the interposing of precisely dimensioned layers of low thermal conductivity between thermistor and heat sink is not claimed in the present application but forms the subject matter of the copending applications of Wormser and De Waard,

Serial No. 528,797, filed August 16, 1955 now Patent No.

2,963,673 and Wormser and De Waard, Serial No. 528,- 798, tiled August 16, 1955 now Patent No. 2,963,674. lt is, however, an important advantage of the present invcntion that very thin reliable thermistors can be produced which make for maiiimum sensitivity.

From the alnve discussion it might be thought that there would be no limit to the increase in sensitivity of the detectors by making the germanium or silicon films thinner and thinner. There is, however, a manufacturing limit on thicknessbelow which reliable and uniform thermistors cannot be assured unless the thermistor is uniform it cannot, of course, have uniform electronic conductivity. This limit, however, is at least ,an order of magnitude thinner than anything which has been possible with the metal oxide thermistoxs which were used in the past.

As has been pointed `out above, the most striking advantages of the present invention are obtained with selective immersed thermistor bolorneters which could not be produced at all with materials hitherto used. Selective thermistor bolometers were developed using coatings or layers on a thermistor of material which absorbed in cer tain bands of the infrared. When other infrared radiation was, kept from heating the thermistor, for example by suitable reflecting coatings, -a detector was produced which responded substantially only to the wavelength I' band in which this selective absorbing coating absorbed.

Only radiation in this band resulted in heating. Selective thermistor bnlometers as such are not a part of the ypresent invention, but constitute the subject matter of the application of Barnes, Wormser and De Waard, Serial No. 641,957, filed February 25, 1957v now Patent No. 2,981,913. It was impossible in the past to produce selective thermistor bolometeis which were immersed. If the n absorbing layer was next to the lens immersion was lost,

and if the thermistor was next to the lens infrared radiation did not penetrate to the absorber because they thermistor material hitherto used was substantially opaque to the infrared.

- All this is changed by the present invention because the thin germanium flakes which are used `in the present 4 invention are transparent to the infrared from about `1` to beyond 25p. In the case of thermistors of silicon the grade of purity now available this transparency is i complete as there are some absorption bands in the inf 5' red even for thin ilrns of silicon. Therefore, wh i transparency over a very wide range isl necessary g manium therrnistors arepreferredin spit of the mi higher temperature coefficient of silic Howev -wherc the infrared response is within the regionsfor wh The infrared rays carry straight through thethin g .manium or silicon thermistor and are selectively absorl in the absorbing coating which can be enhanced by p viding an additional mirror coating which again reflects rays through the selective absorber, the transparent-th r mistor and the lens. The present inventionthus for i first time permits a selective immersed thermistor bolor ter and opens up the whole field of selective therms bolometers to the added sensitivityof immersion.

It should be noted that the lens in the case of immersedtherrnistor bolometer'acts as a-heat sink as vv as a lens, and that the reflecting coatings are 4on the posite side of the thermistor. They are, therefore, tirely unaffected'oy any optical requirements other t1 their absorption spectrum. Thispermits great exibi and allows for the use of any materials so that the pro absorption can be obtained almost anywherel in tlie't able infrared spectrum. Of course, where nonselec thermistor bolometers are required, whether immerse:`

3o unimmersed, the germanium or'silicon thermistor n be blackened. This is a common procedure even t oxide thermistors and the conventional blackening agi such as gold black,-platinum black and the like, x be used.

The invention will be described in greater vdetai` conjunction with the drawings in which:

FIG. 1 is a diagrammatic cross section through'a se tive immersed thermistor bolometeryand FIG. 2 is a composite infrared spectrum showing 4o typical absorption bands for different materials.

EIG. 1 shows an infrared lens 1 of suitable matt such as sapphire, germanium, silicon and the like, a manium or silicon thermistor 2, an 'insulating laye (necessary in the case of electricallyconductive le such as those of germanium), a selective absorbing c ing 4 and a mirror coating 5. For clearness thc t mistor and the various layers are not to scale, the ther tor and the insulating layer being enormously exagger in thickness as is the mirror layer. The electrical lcon tions to the thermistor are not shown 'as these are con tional and are of exactly the same design as with t mistors of the ordinary metal oxide material. The la 2, 3, 4 and 5 are extremely thin. Therefore, it would be possible to show them clearly to scale. Accordir the layers, which are represented by lines, are sh slightlyexploded in FIG. 1. In an actual instrument layers are in intimate contact andv there are no air sp between them.

In discussion .of the operation of the selective mme bolometer we will assume, for example, thatthe abs ing layer is'a thin quartz or glass layer. Infraredstriking the lens l are transmitted from about 1.8, tc yond 25p. The transparent thermistor which may b germanium is transpax ent through the whole of this r;

and in fact even beyond. the long wave extreme. Tl

fore, the infrared radiation from 1.8 to beyond passes through the thermistor and enters the absor layer 4. There is strong absorption in the narrow l of about 8 to 10p as shown in FIG. 2, the other passing through, striking the mirrors and again pa.' through the absorbing layer, the thermistor and through thelens. Further absorption in the absor] band takes place in this reflected path, andas a r the absorbing layer is heated only by the infrared radi:

within its absorption band. It rises in temperalrr this temperature is transmitted by conduction to the thermister. Operation as a selective thermistor bolometer results and by the use of any suitable absorbing layer the bolometer can be made responsive to a preselected band v of infrared radiation. For example, if the absorbingV layer 4 is a thin film of an organic polymer the absorpv uniform layer'of material of low conductivity betwee the thermistor and the heat sink, the thermal impedance presented by the layer being very large compared to the h'eat capacity of the thermistor. l.

2. A thermistor bolometer according toelam 1 in which the thermistor material is gemtanium.

tion will be in the short wave band at about 2.5# shown in FIG. 2.

The time, constant of the system will normally be ff determined by the absorbing layer as usually the heat capacity of the thermistor will be so low and its diffusitivity so high, that it does not limit the time constant.

YWith many absorbing' layers it is' not possible to reach the extremely short time ccnstants'measured in 'microseconds which is possible with the thermistor alone when it is only blackened and the time constant is determined ,by its heat capacity and difusivity.

It will be seenihat the present invention forthe tirst time permits an immersed selective thermistor bolometer and also pemiits bolometers such as immersed thermistor bolometers of non selective type with extraordinarly low time constants. At thel same time these new fields which i are opened to thermistor bolometers are achievedwithout drawbacks. The germanium or silicon thermistors are not significantly more expensive or difiicult to produce than the usual metal oxide thermistors and there is.

no degradation of quality. ln `fact the resulting bolometers may be of even lower noise than the usual type.

ln the drawings there has been illustrated a hemispherical immersion lens. The present invention is applicable equally to any type of immersion lens including hypen'mmersion lenses where the thermistor is located. beyond the center of curvature of the spherical surface. For certain instruments these hyperimmersion lenses present particular advantagesand it is a tribute to the flexibility of the present invention that it is useful in any design of immersed thermistor bolometer.

What I claim isf 3. A thermistorbolorneter according to claim 1 in" which the thermis'tor material is silicon.

4. A thermistor bolometer according to claim l in which the heat sink is a lens 'of infrared transparent 4 material having high heat conductivity, the thermistor is A' prising in combination and in optical alignment a thinV immersed on said lens andthe infrared absorbing mate- 'rial is in contact with'the unimmersed face of the thermistor.

6. A thermistor bolometer of low time constant comthermistor of metallic silicon of' uniform electronic comA ductivity and of a thickness to be transparent to infrared ,radiation longer than its short wavelength cutoff, and -electric connections to said uniformly electrically .con- .ducting thermistor, a window thereover transparent Ithrough* a wide range of the infrared spectrum, an infral red absorbing layer 'In contact with the thermistor and a heat sink in heat'conducting contact'with the thermistor.

7.- A thermistor bolometer of low time constant' com prising in combination and in optical alignment a thin l. A thermistor bolometer of high sensitivity and coning contact with the thermistor, a heatsink and a precisely t layer of germanium of-uniform electronic conductivity andof a thickness to be transparent to infrared radiation longer thanits short 'wavelength cutot, and electric connections. to said uniformly electrically conducting thermistor as a thermistor, a window thereover passing a portion of the infrared spectrum and an absorbing coating on the face of the therrnis'tor opposite the window.

8. A selective thcrrnistor bolometer according to claim i 7 in which the absorbing coating absorbs selectively at least one predetermined band in the infrared.

9. A therrnistor4 bolonietr according to claim 6 inl which the absorbing coating is on the side of the ther- -mistor vopposite the window.

10. A -thermistor bolometer according to claim 9 in which the absorbing coating selectively absorbs at least one band in the infraed. Y

References Cited in the tile of this patent UNITED STATES PATENTS 2,788,381 Baldwin Apr. 9, i951 5.L A thermistor bolometer according to 'claim Y4 in u which both the lens and the thermistcr-are of germanium. 

